PKCι and PKCζ (accession numbers NM_002740 and NM_002744 respectively) together define the atypical sub-class of the protein kinase C(PKC) family. The aPKCs are structurally and functionally distinct from the other PKC sub-classes, classic/conventional and novel, as their catalytic activity is not dependent on diacylglycerol and calcium (Ono, Y., Fujii, T., Ogita, K., Kikkawa, U., Igarashi, K., and Nishizuka, Y. (1989). Protein kinase C zeta subspecies from rat brain: its structure, expression, and properties. Proc Natl Acad Sci USA 86, 3099-3103). Structurally, PKCι and PKCζ contain a C-terminal serine/threonine kinase domain (AGC class) and an N-terminal regulatory region containing a Phox Bem 1 (PB1) domain involved in mediating protein:protein interactions critical for aPKC function. At the amino acid level the aPKCs share 72% overall homology, however, the kinase domains share 84% identity and differ in the active site by just a single amino acid. This striking homology suggests an ATP-competitive ligand would not be expected to exhibit significant aPKC isoform selectivity.
The aPKCs have been implicated in a diverse number of signalling pathways, demonstrating both redundant and distinct signalling functions. Both isoforms have emerged as central players in the mechanisms that regulate the establishment and maintenance of cellular polarity in multiple cell types (reviewed in Suzuki, A., and Ohno, S. (2006). The PAR-aPKC system: lessons in polarity. J Cell Sci 119, 979-987). Genetic dissection of their functions using knockout mice have also revealed preferential roles for PKCζ in the regulation of NF-kB signalling (Leitges, M., Sanz, L., Martin, P., Duran, A., Braun, U., Garcia, J.F., Camacho, F., Diaz-Meco, M. T., Rennert, P. D., and Moscat, J. (2001). Targeted disruption of the zetaPKC gene results in the impairment of the NF-kappaB pathway. Mol Cell 8, 771-780), and PKCι in insulin secretion and action (Farese, R. V., Sajan, M. P., Yang, H., Li, P., Mastorides, S., Gower, W. R., Jr., Nimal, S., Choi, C. S., Kim, S., Shulman, G. I., et al. (2007). Muscle-specific knockout of PKC-lambda impairs glucose transport and induces metabolic and diabetic syndromes. J Clin Invest 117, 2289-2301). In addition, both isoforms have been implicated in the pathogenesis of cancer making a strong case for the inhibition of the aPKCs as a novel therapeutic avenue.
PKCι is a known oncogene in non-small cell lung cancer (NSCLC). In one study it was shown to be overexpressed in 69% of NSCLC cases at the protein level. Consistent with this, the PKCι gene (PRKCI residing on chromosome 3q26) was shown to be amplified in 36.5% of NSCLC tumours examined, including 96% of the squamous cell carcinoma sub-type (Regala, R. P., Weems, C., Jamieson, L., Khoor, A., Edell, E. S., Lohse, C. M., and Fields, A. P. (2005b). Atypical protein kinase C iota is an oncogene in human non-small cell lung cancer. Cancer Res 65, 8905-8911). Amplification of 3q26 has also been reported in 44% of ovarian cancers, including >70% of serous epithelial ovarian cancers where 3q26 amplification is translated into increased PKCζ protein expression. Moreover, increased PKCι expression is associated with poor prognosis in NSCLC and ovarian cancer where it may serve as a diagnostic biomarker of aggressive disease (Eder, A. M., Sui, X., Rosen, D. G., Nolden, L. K., Cheng, K. W., Lahad, J. P., Kango-Singh, M., Lu, K. H., Warneke, C. L., Atkinson, E. N., et al. (2005). Atypical PKCiota contributes to poor prognosis through loss of apical-basal polarity and cyclin E overexpression in ovarian cancer. Proc Natl Acad Sci USA 102, 12519-12524; Zhang, L., Huang, J., Yang, N., Liang, S., Barchetti, A., Giannakakis, A., Cadungog, M. G., O'Brien-Jenkins, A., Massobrio, M., Roby, K. F., et al. (2006). Integrative genomic analysis of protein kinase C(PKC) family identifies PKCiota as a biomarker and potential oncogene in ovarian carcinoma. Cancer Res 66, 4627-4635). 3q26 amplifications have been observed in many other cancers including oesophageal squamous cell carcinoma (Yang, Y. L., Chu, J. Y., Luo, M. L., Wu, Y. P., Zhang, Y., Feng, Y. B., Shi, Z. Z., Xu, X., Han, Y. L., Cai, Y., et al. (2008). Amplification of PRKCI, located in 3q26, is associated with lymph node metastasis in esophageal squamous cell carcinoma. Genes Chromosomes Cancer 47, 127-136) and breast cancer (Kojima, Y., Akimoto, K., Nagashima, Y., Ishiguro, H., Shirai, S., Chishima, T., Ichikawa, Y., Ishikawa, T., Sasaki, T., Kubota, Y., et al. (2008). The overexpression and altered localization of the atypical protein kinase C lambda/iota in breast cancer correlates with the pathologic type of these tumors. Hum Pathol 39, 824-831) suggesting that PKCι may also participate in the pathogenesis of these diseases.
In NSCLC the primary function of PKCι is to drive transformed growth via a Rac1/PAK/MEK/ERK signalling axis. However, PKCι also functions in NSCLC survival, resistance to chemotherapy, and invasion via distinct pathways (reviewed in Fields, A. P., and Regala, R. P. (2007). Protein kinase C iota: human oncogene, prognostic marker and therapeutic target. Pharmacol Res 55, 487-497). In ovarian cancer transformed growth is correlated with deregulated epithelial cell polarity and increased cycle E expression (Eder et al., 2005) suggesting that PKCι can influence the cancer phenotype through multiple mechanisms. Compelling evidence has emerged to suggest that inhibition of PKCι may be a useful therapeutic approach to combat tumours characterised by increased PKCι expression. In transgenic models, mice with elevated PKCι activity in the colon are more susceptible to carcinogen-induced colon carcinogenesis, and expression of a kinase-dead mutant of PKCι blocks the transformation of intestinal cells by oncogenic Ras (Murray, N. R., Jamieson, L., Yu, W., Zhang, J., Gokmen-Polar, Y., Sier, D., Anastasiadis, P., Gatalica, Z., Thompson, E. A., and Fields, A. P. (2004). Protein kinase Ciota is required for Ras transformation and colon carcinogenesis in vivo. J Cell Biol 164, 797-802). Finally, genetic or pharmacological inhibition of PKCι by a gold derivative—aurothiomalate (ATM)—blocks the growth of NSCLC cells in soft agar and significantly decreases tumour volume in xenograft models of NSCLC(Regala, R. P., Thompson, E. A., and Fields, A. P. (2008). Atypical protein kinase C iota expression and aurothiomalate sensitivity in human lung cancer cells. Cancer Res 68, 5888-5895; Regala, R. P., Weems, C., Jamieson, L., Copland, J. A., Thompson, E. A., and Fields, A. P. (2005a). Atypical protein kinase Ciota plays a critical role in human lung cancer cell growth and tumorigenicity. J Biol Chem 280, 31109-31115).
Despite the high degree of similarity between aPKC isoforms, the role of PKCζ in cancer is distinct from that of PKCι. PKCζ plays a role in NSCLC cell survival by phosphorylating and antagonising the pro-apoptotic effects of Bax in response to nicotine (Xin, M., Gao, F., May, W. S., Flagg, T., and Deng, X. (2007). Protein kinase Czeta abrogates the proapoptotic function of Bax through phosphorylation. J Biol Chem 282, 21268-21277). PKCζ activity has also been linked to resistance against a wide range of cytotoxic and genotoxic agents. For instance, in human leukaemia cells, overexpression of PKCζ confers resistance against 1-β-D-arabinofuranosylcytosine (ara-C), daunorubicin, etoposide, and mitoxantrone-induced apoptosis (Filomenko, R., Poirson-Bichat, F., Billerey, C., Belon, J. P., Gamido, C., Solary, E., and Bettaieb, A. (2002). Atypical protein kinase C zeta as a target for chemosensitization of tumor cells. Cancer Res 62, 1815-1821; Plo, I., Hernandez, H., Kohlhagen, G., Lautier, D., Pommier, Y., and Laurent, G. (2002). Overexpression of the atypical protein kinase C zeta reduces topoisomerase II catalytic activity, cleavable complexes formation, and drug-induced cytotoxicity in monocytic U937 leukemia cells. J Biol Chem 277, 31407-31415). Furthermore, inhibition of PKCζ activity through expression of a kinase-dead mutant sensitises leukaemia cells to the cytotoxic effects of etoposide both in vitro and in vivo (Filomenko et al., 2002). Atypical protein kinase C regulates dual pathways for degradation of the oncogenic coactivator SRC-3/AIB1. Mol Cell 29, 465-476), and both of these proteins have been postulated to play a role in tamoxifen resistance in breast cancer (Iorns, E., Lord, C. J., and Ashworth, A. (2009). Parallel RNAi and compound screens identify the PDK1 pathway as a target for tamoxifen sensitization. Biochem J 417, 361-370; Osborne, C. K., Bardou, V., Hopp, T. A., Chamness, G. C., Hilsenbeck, S. G., Fuqua, S. A., Wong, J., Allred, D. C., Clark, G. M., and Schiff, R. (2003). Role of the estrogen receptor coactivator AIB1 (SRC-3) and HER-2/neu in tamoxifen resistance in breast cancer. J Natl Cancer Inst 95, 353-361). Together these studies suggest that inhibition of PKCζ activity may have beneficial therapeutic effects by acting as a chemosensitiser to a wide array of commonly used chemotoxic agents in the clinic.
Further evidence that small molecule inhibition of PKCζ could have important therapeutic benefits has recently emerged from tumour models that link PKCζ signalling to the mTOR pathway. PKCζ is constitutively activated in follicular lymphoma and has been identified as a novel target for the anti-CD20 therapeutic antibody rituximab (Leseux, L., Laurent, G., Laurent, C., Rigo, M., Blanc, A., Olive, D., and Bezombes, C. (2008). PKC zeta mTOR pathway: a new target for rituximab therapy in follicular lymphoma. Blood 111, 285-291). Rituximab inhibits follicular lymphoma proliferation by targeting a PKCζ-MAPK-mTOR pathway, suggesting that PKCζ is both a target of Rituximab, and a key regulator of its' anti-leukaemic effect. Regulation of the mTOR/p70S6K pathway by PKCζ has also been implicated in the transition of prostate cancer cells to an androgen-independent state (Inoue, T., Yoshida, T., Shimizu, Y., Kobayashi, T., Yamasaki, T., Toda, Y., Segawa, T., Kamoto, T., Nakamura, E., and Ogawa, O. (2006). Requirement of androgen-dependent activation of protein kinase Czeta for androgen-dependent cell proliferation in LNCaP Cells and its roles in transition to androgen-independent cells. Mol Endocrinol 20, 3053-3069). Finally, mice containing a homozygous deletion of Par4, a negative regulator of PKCζ, exhibit greatly enhanced PKCζ activity. These mice spontaneously develop tumours of the prostate and endometrium, and potentiate Ras-induced lung carcinogenesis consistent with a role for PKCζ in lung cancer (Garcia-Cao, I., Duran, A., Collado, M., Carrascosa, M. J., Martin-Caballero, J., Flores, J. M., Diaz-Meco, M. T., Moscat, J., and Serrano, M. (2005). Tumour-suppression activity of the proapoptotic regulator Par4. EMBO Rep 6, 577-583; Joshi, J., Fernandez-Marcos, P. J., Galvez, A., Amanchy, R., Linares, J. F., Duran, A., Pathrose, P., Leitges, M., Canamero, M., Collado, M., et al. (2008). Par-4 inhibits Akt and suppresses Ras-induced lung tumorigenesis. EMBO J. 27, 2181-2193).